Both the same. They're both black = pain in the ass to keep clean, but looks stunning when washed and waxed. Sapphire Black is a metallic color, so it looks a bit nicer. Jet black is non-metallic, and looks much deeper when clean/waxed. Can't go wrong with either!
'12 M3 6MT Alpine White + Fox Red Extended | '09 328i 6MT Space Gray + Black